Australian authorities charged a teenager and four men Thursday with planning terrorist attacks against government buildings, including the headquarters of the Australian Federal Police in Sydney, and against random civilians.

Earlier in the day, police arrested a 15-year-old boy and a 20-year-old man from western Sydney and charged them with “conspiracy to prepare for a terrorist act,” Australian Federal Police Deputy Commissioner Michael Phelan said to reporters.

"Since December, our detectives within that task force have been working tirelessly to put together all of the evidence as a result of the search warrants that were executed on those days in December as well as numerous amounts of electronic surveillance and physical surveillance activity, have today culminated in what we believe is sufficient evidence to be able to put before the court to charge this morning, two individuals with conspiracy to prepare for a terrorism act," Phelan said.

Three other men, already in jail on separate terrorism-related charges, were charged with being part of the plot, police said. If convicted, all face a potential sentence of life in prison.

‘Very serious offence’

Catherine Burn, a state police deputy commissioner said at the same press conference that it was disturbing to see a teenager charged as an adult with a “very serious offence.”

"It's disturbing that we continuing to see teenage children in this environment and there's absolutely no doubt that the fact that we are charging a 15-year-old with a very, very serious offense. This is an offense that has a maximum of life imprisonment. This is concerning, not only to us in law enforcement but this should be concerning to everybody," Burns said.

Australian police have regularly conducted raids across the country since the government raised the nation's terrorism alert level in September 2014 in response to the domestic threat posed by homegrown radicals as supporters of Islamic State.